Method for producing an optical transmitting and receiving device and a transmitting and receiving device produced according to said method

ABSTRACT

The invention relates to a method for producing an optical transmitting and receiving device ( 1, 1   a ) comprising a light emitting transmission element ( 3, 3   a ) and a receiving element ( 4, 4   a ) which converts this light into an electrical magnitude. The transmission and receiving elements are inserted into a silicon substrate. The optical transmitting and receiving device ( 1 ) is preferably inserted in a monolithic manner into a common substrate, comprising a sequence of superimposed layers for the light emitting transmission element ( 3 ) and the light receiving element ( 4 ). An electrically insulating intermediate layer ( 9, 9   a ) is incorporated between the transmission and receiving element.

[0001] The invention relates to a method for producing an opticaltransmitting and detecting device including a light-emittingtransmitting element as well as a detecting element to convert thislight into an electrical quantity, the transmitting and detectingelement being incorporated into a silicon substrate.

[0002] In addition, the invention relates to the transmitting anddetecting element produced based on the method according to theinvention.

[0003] A known approach exists in which this type of transmitting anddetecting system is arranged sequentially on a frame to form anoptocoupler, but the approach is complex due to the associated processof bending and precise assignment. Electrical insulation is createdbetween the transmitting and detecting systems by providing, forexample, an intermediary plastic or lacquer or similarlyoptically-transparent material.

[0004] The publication by S. M. Sze, Physics of Semiconductor Devices,2^(nd) edition, page 699 discloses an optocoupler which has an LED asthe optical transmitting element and a silicon phototransistor as thelight detector arranged on both sides of a glass insulator.

[0005] Here again, a complex assembly technology is required both forproduction and for aligning an optical coupling which significantlyaffects the cost of production.

[0006] Optical transmitting elements (LEDs) are constructed on the basisof gallium arsenide, gallium arsenide phosphide, or gallium phosphidedue in part to the high efficiency achieved. A disadvantage of thesecompound semiconductors is, however, their poorer mechanical propertiesas compared to silicon as well as the problem of integration intosilicon-based systems.

[0007] To integrate optical systems based on GaAs, GaAsP, or GaP, theapproach has also been suggested of incorporating a receptacle recess inthe silicon substrate in which the optical system is then inserted and,for example, joined to the silicon system by bonding. An optical linkcan then be created, for example, by lateral emission from the opticalgallium arsenide system to the silicon system.

[0008] This approach too requires a complex assembly technology.

[0009] The publication Sensors and Actuators A 31 (1992) pp. 229-240discloses an optocoupler in which the light source is an avalanchesilicon diode.

[0010] The optocoupler has arranged adjacent to one another on asubstrate the avalanche diode as the light source and the light detectorin the form of a photodiode. This arrangement requires a comparativelylarge chip surface area. In addition, the transmitter and detector arenot dielectrically separated. Finally, certain measures are required tofacilitate the lowest-possible-loss transmission of light between thetransmitter and the detector—another factor increasing the complexity ofproduction.

[0011] The goal of the invention is to create a method of the typedescribed at the outset as well as an optical transmitting and detectingdevice produced based on the method, wherein low-cost production ispossible and a reduced requirement for chip area is provided, andwherein a multiplicity of applications is opened up.

[0012] To achieve this goal, it is proposed that the opticaltransmitting and detecting device is monolithically incorporated into acommon substrate having a stacked sequence of layers for thelight-transmitting element and the light-detecting element, and that anelectrically insulating interlayer is inserted between the transmittingelement and the detecting element.

[0013] Based on the sandwiched stacked transmitting and detectingelements, the space requirement for the chip area is considerablyreduced (almost to half), while the spacing between the two elements isadditionally kept as small as possible, thereby minimizing transmissionlosses, or significantly enhancing transmission efficiency. As a result,the power of the transmitting element may be very small without thisnegatively affecting functionality. Specifically, the reduced lightintensity of the silicon-based transmitting element as compared togallium arsenide systems is sufficient for reliable functioning evengiven low operating currents.

[0014] Another advantage is the electrical separation of the systems bythe insulating interlayer—a required feature when using the device, forexample, as an optocoupler.

[0015] In this first proposed approach for producing an opticaltransmitting and detecting device according to the invention, thetransmitting and detecting elements are incorporated into a commonsubstrate.

[0016] According to another independent proposed solution, thelight-transmitting element and light-detecting element may each beincorporated into a silicon substrate, wherein at least one of the twotransmitting and detecting chips thus formed is modified on thelight-emitting or light-detecting side, this modification involving theinsertion of a cavity into at least one of these sides, and the chipsbeing joined with the light-emitting and the light-detecting sidesfacing each other.

[0017] The preferred use of this method is when modifications must bemade between the transmitting and the detecting element such as creatinginterlayers, inserting a cavity, and the like. Since initially twosystems are present on separate substrates, those sides which will laterbe joined and face each other are more easily accessible for suchmodifications—with the result that this production method offerssimplification and advantages in terms of special modifications.

[0018] Preferably, the transmitting chip and detecting chip may bejoined in sandwiched stacked form by bonding. Chip bonding is a commonmethod available in a number of variants.

[0019] The invention relates to an optical transmitting and detectingdevice produced based on the method according to the invention includinga light-emitting transmitting element as well as a detecting element toconvert this light into an electrical quantity, the transmitting and thedetecting element being incorporated into a silicon substrate.

[0020] In a first embodiment of the invention, the transmitting anddetecting elements are arranged stacked in a common silicon substrate,and a dielectric interlayer is arranged between the transmitting elementand the detecting element.

[0021] In a second embodiment of the invention, one silicon chip each isprovided for the light-emitting element and for the light-detectingelement, these chips being joined with the light-emitting and thelight-detecting sides facing each other, and at least one cavity and/orat least one insulating layer and/or at least one wavelength-selectivelayer being provided.

[0022] The two embodiments have in common that the overall systemsrequire a small chip area and that a very short transmission path hasbeen realized.

[0023] The second embodiment allows for simplified insertion of apreferably externally-accessible cavity, or of interlayers. An extremelycompact analysis system is thereby created which may be employed toanalyze liquids and gases present in the cavity.

[0024] Additional embodiments of the invention are listed in subsequentsubclaims. The invention with its essential details is explained belowwith reference to the drawings, the drawings being in essentiallyschematic form.

[0025]FIG. 1 is a cross section through an optical transmitting anddetecting device.

[0026]FIG. 2 is a cross section through an optical transmitting anddetecting device which includes a cavity in the light transmission path.

[0027] An optical transmitting and detecting device 1 shown in FIG. 1has, on a substrate 2, a sequence of layers for a transmitting element3, and located above said element a sequence of layers for a detectingelement 4.

[0028] To create transmitting element 3, an epitaxial layer 5 of asecond doping type is applied to the heavily-doped silicon substrate 2of a first doping type. The blocking layer between these two layers isthe region of light emission.

[0029] After an intrinsic layer 6 has been applied to epitaxial layer 5,a layer 10 of the same doping type is diffused in, and extends up toepitaxial layer 5 of transmitting element 3. This the first pole. In theembodiment, the anode of transmitting element 3 is routed to the topside of the chip.

[0030] In this embodiment, the metallized bottom side of siliconsubstrate 2 forms the cathode terminal 20 of transmitting element 3.This terminal contact may also be routed to the top side if required.

[0031] In the embodiment, detecting element 4 is formed by a pin diodeincluding intrinsic layer 6 and diffusion zones 7, 8 spaced laterallyrelative to each other therein. To produce detecting element 4,intrinsic layer 6 is applied to epitaxial layer 5 of the transmittingelement, and the two zones 7 and 8 are diffused in. To obtain theelectrical insulation of transmitting element 3 from detecting element4, a dielectric interlayer 9, 9 a is inserted between these elements.The interlayer is formed by an oxide layer.

[0032] The horizontal interlayer section 9 is formed byoxygen-implantation according to the SIMOX process.

[0033] The lateral interlayer sections 9 a are formed by trench etchingand trench sealing.

[0034] The top side of intrinsic layer 6 is provided with an oxide layer11 as a protective coating. Finally, the surface is provided through aconventional bonding process with terminal contacts, the metallized topside of layer 10 forming the anode terminal 12 of transmitting element3. The metalized top sides of the two diffusion zones 7 and 8 form thecathode terminal 13 and the anode terminal 14 for detecting element 4. Aclearly seen feature is that anode terminal 14 extends over the rearside of detecting element 4, thereby forming a metal layer acting as areflector 15. This metal layer enhances the efficiency of the detectingelement.

[0035] Reflector 15 may also be isolated from the other metallic contactcoatings.

[0036] If multiple transmitting and detecting devices 1 are arrangedsequentially on a common substrate, an oxide layer 16 is provided forsystem isolation, the layer being formed by trench etching or trenchsealing and extending from the top side of the chip beyond blockinglayer 17 of transmitting element 3.

[0037] Oxide layer 16 is added simultaneously with interlayer 9 a.During the trench-etching process, etching is halted when the etchingagent (etching gas) reaches the horizontal silicon oxide layer 9. Theadjacent external trenches, which are produced at the same time, do notmeet this type of silicon oxide layer so that they can be made deeperand extend beyond blocking layer 17, thus creating the system isolation.

[0038] The arrangement of transmitting element 3 and detecting element 4with its sandwiched stacked layering creates a very short lighttransmission path in which practically only insulating interlayer 9 islocated. The very short transmission path means that only a very lowintensity is required for the light emitted by transmitting element 3,that is, transmitting element 3 may be operated with very low currents.

[0039] Detecting element 4 which is formed by a pin diode has anextremely high light sensitivity, and thus a high efficiency. This alsocontributes to the fact that the transmission side may be operated at avery low light output. This aspect is aided by the metallic layer actingas reflector 15 on the rear side of detecting element 4. As withcontacts 12 through 14, this layer may be composed of aluminum, or alsopossibly of gold or another metal with high light reflectivity.

[0040] Transmitting and detecting device 1 may be provided sequentiallyin a multiple-element design in order to create a multichannelarrangement for the light transmission.

[0041] It must also be mentioned that detecting element 4 may also be inthe form of a phototransistor, photothyristor, photoresistor or similarlight-sensitive element. It is also possible on the detecting side tointegrate a following series-connected power switch so as to create, forexample, a photo MOS relay.

[0042] The transmitting and detecting device 1 in FIG. 1 may be employedas an optocoupler for example.

[0043]FIG. 2 is a modified embodiment of a transmitting and detectingdevice I a according to the invention. This embodiment first providesisolated, separate silicon chips for the transmitting element 3 a anddetecting element 4 a. After their production, these are joined inbonding region 18 by chip bonding, with the light-emitting andlight-detecting sides facing each other, thereby producing an embodimentapproximately comparable to the embodiment of FIG. 1, whereinspecifically here as well the layers of transmitting element 3 and thelayers of detecting element 4 are arranged in a sandwiched stackedconfiguration.

[0044] In the embodiment of FIG. 2, a cavity 19 is located in the lighttransmission path between transmitting element 3 a and detecting element4 a, the cavity being preferably accessible from the outside. In theembodiment, cavity 19 is formed by removing silicon from the epitaxiallayer 5. Since this side of the transmitting chip is freely accessiblebefore bonding to the detecting chip, cavity 19 may be produced simply.The insulating interlayer 9 formed by the oxide layer may also beproduced more easily before bonding due to the accessibility of thedetecting chip side than in the embodiment of FIG. 1. If necessary,insulating interlayer 9 may be dispensed with if a gas is present incavity 19 which creates a sufficient isolation and electrical insulationbetween the systems.

[0045] The coatings in the light transmission path may be provided forreasons other than insulation purposes. One possibility is to create awavelength-selective filter layer, such as one composed of siliconnitride¹, which transmits light only at a wavelength above 400 nm.

[0046] In addition to these wavelength-selective filter layers, almostany number of coatings may be applied to affect light transmissionbetween transmitting element 3 a and detecting element 4 a.

[0047] The externally accessible cavity 19 may be filled or penetratedby a gaseous or liquid media. It is thus possible to determine the typeof material, employ the material as a filter, etc. Analyses of thesemedia may thus be performed in which the measurement reaction occurringis detected based on the specific material. For example, spectrometricanalyses may be performed or certain media monitored for turbidity. Thisfeature would enable monitoring of water quality, for example, or use asa fire detector in which smoke or the changed composition of the ambientair affects the transmission of light.

[0048] The arrangement of transmitting element 3 a, cavity 19 anddetecting element 4 a thus create a very compact analysis system in apreferably constructed chip.

[0049] It should be mentioned that a cavity in the light transmissionpath may also be provided in the embodiment of FIG. 1, one created forexample by underetching. Due to higher production costs for theembodiment of FIG. 1, preferably one or more microcavities are provided,whereas in the embodiment of FIG. 2 with its accessibility to the innersides of the two chips, one or more cavities of any size and shape maybe provided.

[0050] It should also be mentioned here that the cavity 19 may beincluded not only in the transmitting chip but also in the detectingchip.

[0051] Since transmitting and detecting device 1 and 1 a are producedbased on silicon technology, they may be readily integrated into othersilicon-based systems. Since the systems are largely protected fromambient light, flip-chip assembly is also possible.

1. Method for producing an optical transmitting and detecting device (1,1 a) including a light-emitting transmitting element (3, 3 a) as well asa detecting element (4, 4 a) to convert this light into an electricalquantity, the transmitting and detecting element being incorporated intoa silicon substrate, characterized in that the optical transmitting anddetecting device (1) is monolithically incorporated into a commonsubstrate having a stacked sequence of layers for the light-transmittingelement (3) and the light-detecting element (4), and that anelectrically insulating interlayer (9, 9 a) is inserted between thetransmitting element and the detecting element.
 2. Method according toclaim 1, characterized in that an epitaxial layer (5) of as second typeis applied to a heavily doped silicon substrate (2) of a first dopingtype, thus creating a light-transmitting element (3, 3 a).
 3. Methodaccording to claim 2, characterized in that a sequence of layers isapplied to the epitaxial layer (5) of the light-transmitting element (3)to form a detecting element (4), and that a dielectric interlayer (9, 9a) is inserted between the layers of the light-transmitting element andthe layers of the detecting element.
 4. Method according to claims 2 or3, characterized in that an intrinsic layer (6) is epitaxially appliedto the epitaxial layer (5) of transmitting element (3) to form adetecting element (4), into which intrinsic layer laterally spaced zones(7, 8) of different doping types are diffused.
 5. Method according toone of claims 2 through 4, characterized in that layers (9 a) of thesecond doping type are each diffused laterally into the layer of thedetecting element (4), the layers extending up to the epitaxial layer(5) of the same doping type of the transmitting element (3).
 6. Methodaccording to one of claims 1 through 5, characterized in that a firstoxide layer forming the dielectric interlayer (9) is created by oxygenimplantation (SIMOX method), and that subsequently a second oxide layer(9 a) extending up to the surface and surrounding the detecting element(4) is created by trench etching or trench sealing.
 7. Method accordingto one of claims 1 through 6, characterized in that a multiplicity oftransmitting and detecting systems (1) are produced adjacent to oneanother on a wafer.
 8. Method according to one of claims 5 through 7,characterized in that preferably an oxide layer (16, 16 a) extendingfrom the top side of the chip and beyond the blocking layer (17) of thetransmitting element (3) is created to separate adjacent transmittingand detecting systems, specifically by trench etching and trenchsealing, simultaneously with the incorporation of the oxide layer (9, 9a) surrounding the detecting element (4).
 9. Method according to one ofclaims 1 through 8, characterized in that at least one, possiblyexternally-accessible cavity (19) is incorporated specifically by theprocess of underetching between the sequence of layers of thelight-transmitting element (3) and the sequence of layers of thelight-detecting element (4).
 10. Method according to the preamble ofclaim 1, characterized in that the light-transmitting element (3 a) andthe light-detecting element (4 a) are each incorporated into a siliconsubstrate, that at least one of the two thus-formed transmitting anddetecting chips is modified at the light-emitting or light-detectingside, that through this modification a cavity (19) is inserted in atleast one of these sides, and that the chips are joined with thelight-detecting sides facing each other.
 11. Method according to claim10, characterized in that the transmitting chip and the detecting chipare joined by bonding to each other in a sandwiched stackedconfiguration.
 12. Method according to claims 10 or 11, characterized inthat, to create the transmitting chip, an epitaxial layer (5) of asecond doping type is applied to a heavily doped silicon substrate (2).13. Method according to one of claims 10 through 12, characterized inthat a multiplicity of transmitting chips is created on a first wafer,and a multiplicity of detecting chips is created on a second wafer, andthat the two wafers are then congruently joined with their respectivechips by wafer bonding.
 14. Method according to one of claims 10 through13, characterized in that the preferably externally-accessiblecavity/ies (19) is/are inserted in the transmitting chip on the sidewithin the light transmission region facing the detecting chip. 15.Method according to one of claims 9 through 14, characterized in thatthe cavity/ies (19) is/are filled with a gaseous or liquid medium. 16.Method according to one of claims 10 through 15, characterized in thatat least one insulating layer (9) and/or at least onewavelength-selective filter layer, for example, one composed of siliconnitride, is inserted into the sides of the transmitting chips anddetecting chips facing each other.
 17. Method according to one of claims1 through 16, characterized in that the terminal contacts (12, 13, 14,20) for the light-detecting element (4, 4 a) and light-transmittingelement (3, 3 a) are produced by conventional diffusion technology andmetallization.
 18. Method according to one of claims 1 through 17,characterized in that, preferably during metallization of the terminalcontacts, a metal coating acting as a reflector (15) and covering therear side of the detecting element (4) is applied.
 19. Method accordingto one of claims 1 through 18, characterized in that the light-detectingelement (4) is coupled to an integrated power switch.
 20. Opticaltransmitting and detecting device including a light-emittingtransmitting element (3, 3 a) as well as a detecting element (4, 4 a) toconvert this light into an electrical quantity, the transmitting and thedetecting element being incorporated into a silicon substrate producedby a method according to one of claims 1 through 18, characterized inthat the optical transmitting and detecting elements (3, 4) are arrangedin a stacked configuration in a common silicon substrate, and that adielectric interlayer (9, 9 a) is located between the transmittingelement and the detecting element.
 21. Optical transmitting anddetecting device according to the preamble of claim 20, characterized inthat one silicon chip each is provided for the light-transmittingelement (3 a) and for the light-detecting element (4 a), that thesechips are joined with the light-transmitting element and thelight-detecting sides facing each other, and that at least one cavity(19) and/or at least one insulating layer (9) and/or at least onewavelength-selective layer are provided between the transmitting elementand the detecting element.
 22. Optical transmitting and detecting deviceaccording to claims 20 or 21, characterized in that the light-detectingelement (4, 4 a) is a pin diode.
 23. Optical transmitting and detectingdevice according to one of claims 20 through 22, characterized in thatsaid device is designed as an optocoupler, possibly provided with apower switch on the detector side.
 24. Optical transmitting anddetecting device according to one of claims 20 through 23, characterizedin that said device has at least one preferably externally-accessiblecavity (19) between transmitting elements (3, 3 a) and detectingelements (4, 4 a), and is designed specifically as an analysis systemfor spectrometric analyses.
 25. Optical transmitting and detectingdevice according to one of claims 20 through 23, characterized in thatsaid device has at least one externally accessible cavity (19) betweentransmitting elements (3, 3 a) and detecting element (4, 4 a), and isdesigned specifically as a fire detector or to analyze liquids, forexample to analyze water quality.